nevermind, I see what's happening in the UI. Each `jj` change is preserved in the UI and we can see multiple versions of the same change. The stack then is not really a stack of PRs but a stack of changes (where each change has its own history, i.e., the interdiff view). Did I get it mostly right?

yes, thats right! when you submit a branch, you can choose to "stack" it, so the individual commits in the branch turn into separate PRs. these PRs evolve individually, can be merged individually, and be reviewed individually. you can also set different reviewers/labels for different PRs in the stack.